Add the gallery timeline item rendered as a placeholder

Introduces the GalleryRoomTimelineItem and its typed GalleryItem model (reusing the existing per-message content types), builds it in the RoomTimelineItemFactory and routes it through RoomTimelineItemView. For now it renders as a placeholder; the grid/list media views and the full-screen preview land in follow-up PRs.

Co-authored-by: svetoslav.mitov <svetoslav.mitov@sportinno.com>

ElementX/Sources/Services/Timeline/TimelineItems/Items/Messages/GalleryRoomTimelineItem.swift

@@ -0,0 +1,31 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+import UIKit
+
+nonisolated struct GalleryRoomTimelineItem: EventBasedMessageTimelineItemProtocol, Equatable {
+    let id: TimelineItemIdentifier
+    let timestamp: Date
+    let isOutgoing: Bool
+    let isEditable: Bool
+    let canBeRepliedTo: Bool
+    var shouldBoost = false
+    
+    let sender: TimelineItemSender
+    
+    let content: GalleryRoomTimelineItemContent
+    
+    var properties = RoomTimelineItemProperties()
+    
+    var body: String {
+        content.caption ?? content.body
+    }
+    
+    var contentType: EventBasedMessageTimelineItemContentType {
+        .gallery(content)
+    }
+}